This invention is related to bridges. The ultimate goals of an engineer in the design of a bridge are to create a structure which meets the area's demands, lasts the longest, and costs the least.
In recent years, engineers have developed portable bridges to cross short spans of no more than 200 feet. These portable bridges basically include a steel structure that includes a simple beam which crosses the span. Depending upon the loads carried by the beam, the structure is more or less light. The weight and maneuverability of the bridge also depend on the particular design employed.
There are several types of portable bridges, some of which use very well designed systems for forming the beam. Some of these bridges can be self launched using additional parts, while others need false work to be installed. In every instance, these bridges use vertical bearing walls as supports at both sides of the river, chasm or the like.
These bearing walls are buried into the soil as near to the water as possible, in order to shorten the length of the beam. These bearing walls are made of masonry or concrete and are affected by the water. In some cases, the whole bridge has failed because of the effect of floods on these bearing walls. Severe flooding can also separate the portable beam from these bearing walls, since the bridge is simply supported. Nowadays, as a result of the improvement of materials and design procedures, steel is used not only in the superstructure of bridges, but also in the deck and the foundations.
Cable bridges traditionally have used large concrete counterweights buried into the soil to absorb the force carried along the cables. Cable stayed bridges generally use the weight of the back part of the deck to equilibrate the vertical component of the force carried by the front stays, and the deck itself to absorb the horizontal component of the force in the back stays.
As discussed above, in the field of portable bridges, the system of a more or less light-weight simple beam that is assembled across the river supported by vertical bearing walls has been improved during the last years, although there is a need to develop other alternatives to make the bridge lighter and safer.
Statically determined trusses are among the most efficient structures for use in bridges. Therefore, bridges in accordance with the invention use trusses in all of their structural parts.